1. Field of the Invention
The present invention relates to a bicycle fork and steering tube construction with particular application to BMX bicycles.
2. Description of the Prior Art
Bicycles have historically be constructed with a cylindrical steerer tube having an upper end joined to the bicycle handlebars, and a lower end to which a bicycle front wheel fork is rigidly fastened. The steerer tube is rotatable within a hollow, cylindrical head tube which forms a part of the bicycle frame and which is located at the front of the bicycle frame. The rider rotates the steerer tube by turning the handlebars to the left or right in order to turn the front wheel fork to the left or right. By altering the orientation of the front wheel relative to the bicycle frame, the rider is able to change the direction of travel of the bicycle to the left or right as desired.
Conventional bicycle fork and steering tube structures have certain disadvantages. These disadvantages are particularly pronounced in the case of BMX bicycles, in which reduced weight and reduced complexity of construction are particularly important. There are several standard bicycle front wheel fork and steerer tube structures that are widely used in different styles of BMX bicycles.
One prevalent type of bicycle front wheel fork and steerer tube combination is known as the “unicrown” fork. Bicycle forks of this type are comprised of bent tubular fork legs welded directly to the hollow, cylindrical steerer tube. In this type of bicycle fork and steerer tube construction the steerer tube is simply cut off at right angles at its lower end and the fork legs are mitered to meet the sides of the open-ended steerer tube.
The “unicrown” configuration results in a open hole at the base of the steerer tube of the finished fork. This open hole can become fouled with debris that might impede the rotation of the front bicycle wheel that rotates about the front wheel axis within the fork. Furthermore, this structure allows for relatively large, localized deformations under loads, thus resulting in metal fatigue and ultimately metal failure. To avoid metal fatigue, heavy wall thickness are required in both the steerer tube and the fork legs. As a result, the finished bicycle front wheel fork and steering tube combination is excessively heavy.
In order to mount the front bicycle wheel between the fork legs, dropout flanges are typically welded to the lower extremities of the fork legs. These dropout flanges are formed as solid plates, castings, or forgings welded to the end or side of each fork leg. That is, the dropout flanges project in planes parallel to each other that are both perpendicular to the plane of alignment of the legs of the bicycle fork. Due to the off center stresses that result from torque forces acting relative to the plane of alignment of the fork legs, the entire lower fork assembly, specifically the fork legs, the dropout flanges and the welds that join the dropout flanges to the lower ends of the fork legs must be very sturdy and are unduly heavy. Lighter weight dropout flanges tend to be thinner and significantly weaker than desired.
Also, in the field of BMX bicycle forks and steerer tubes it is often necessary to run a front brake cable down the center of the steerer tube in order to activate the front brake while avoiding contact between the brake cable and the frame when the steering system is rotated more than one hundred eighty degrees. The brake cable passage created conflicts with the “top bolt” which is required for setting up the headset that holds the bicycle handlebars and which bears tension.
The conventional way of routing the brake cable is to run it through the center of a hollow bolt having external hexagonal wrench flats that accommodate a wrench for tensioning the bolt. That is, the conventional hollow bolt system relies upon a large, external hexagonal configuration on a low bolt head or a smaller hex head that protrudes upwardly in an undesirable way. Another common solution to this problem is to use a hexagonal socket head located centrally in the bolt. However, with the prevalent existing standard headset sizes there is insufficient room to keep a hexagonal socket head central while running the cable eccentrically down through the hollow bolt. A common solution to this is to run the cable through the hexagonal socket itself, but this renders the socket head unusable until the brake cable has been removed. Furthermore, with the hex head top bolts most widely utilized, it is not possible to pass a brake cable assembly through intact. Therefore, any replacement of headset parts, stem, or frame parts, generally requires a labor intensive disassembly of the front brake cable adjustment setup at the front braking caliper.